In the prior art circuits are known for the thermal protection and for the power regulating as a function of a current winding temperature of phase windings of an electric motor. Traditional circuits comprise on the one hand a temperature monitor switch measuring the winding temperature of the phase windings, which upon reaching a certain winding temperature, the switching temperature, produces a switching off of a winding current flowing through the windings, as well as a temperature-dependent resistor associated with the phase windings, which upon reaching a certain winding temperature brings about, for example, a reduction in the motor power due to a resulting rise in the resistance.
From the documents DE 10 2013 107 819 A1, EP 1 303 021 A2 and EP 2184830 A1 there are known for example a circuit for the thermal protection and power regulation in dependence on a current winding temperature of phase windings of an electric motor.
As is known, the protection of a three-phase electric motor against thermal overload is accomplished by three temperature monitor switches.
The temperature of each motor phase is sensed by its own temperature monitor, in order to also detect asymmetrical temperature rises, e.g., due to a winding insulation fault. The three temperature monitor switches are connected in series. Upon exceedance of the switching temperature of one of the temperature monitor switches, the switch opens. The interruption is detected in an evaluation electronics and the necessary action for the particular protection concept is triggered.
Based on the assumption that, given proper functioning of the motor, the temperatures in the three winding phases are approximately equal, typically only one temperature-dependent resistance is used in order to reduce costs. In order to evaluate the series circuit of the three temperature monitor switches by an evaluation electronics, two connections are needed in the signal electronics. Likewise, for the evaluation of the temperature-dependent resistance by the evaluation electronics, two connections to the electronics are needed. Thus, on the whole, four connections are needed between the electric motor and the evaluation electronics. This results in a higher circuitry and componentry expense.
In traditional motor circuits, accordingly, either a temperature monitor/switch (digital) or a temperature sensor (analog) introduced directly in the motor winding is used to limit the winding temperature. This usually makes contact with two connection elements (contacts, lines) led out from the motor in addition to the three winding terminals.
A digitally operating temperature monitor/switch may also be integrated directly in series with the winding phases in the power pathway. In the former case, the opening of the temperature monitor at excessive motor temperature or the passing of a limit temperature of the analog sensor is detected by the electronics and the motor current is interrupted in this way. This has the drawback, among others, that two additional connection elements are required for the detection between motor and electronics, as well as an additional evaluation circuit in the electronics.
In the second case, the direct switching off at excess temperature occurs by the direct disconnection of the winding phases from the electronics by means of the temperature monitor itself. While this solution requires no additional lines, the temperature monitor must be designed for the secure disconnecting of the entire motor current (even during excess current). Moreover, one drawback is that the monitor must carry the entire motor current during normal operation.